Certain firearms may operate to fire either from an open bolt configuration or a closed bolt configuration.
Open bolt operation proceeds with a trigger pull causing a retracted bolt to move forward, stripping a cartridge from a magazine or belt, chambering the cartridge, and firing the cartridge. In response to firing one or more rounds, the bolt is forced back to the open position, where it is held until the trigger is pulled again. Open bolt arms have the advantage of allowing the barrel and chamber to cool more readily after sustained firing by keeping the chamber open. They avoid the risk of “cook-off,” which occurs when a cartridge is chambered into a very hot chamber, and heated to the point of ignition, causing the rifle to discharge even when the trigger is not pulled and a safety is engaged. Open bolt arms also prevent the lead core of a chambered projectile from melting away from the copper jacket, thus preventing damage to the barrel or barrel mounted sound suppressors.
Open bolt arms suffer the disadvantage of increased susceptibility to dirt and contamination entering the action and chamber, because the bolt is normally open and exposes these areas to the environment. Open bolt arms are considered less accurate for aimed fire, because the abrupt motion of the heavy bolt after trigger pull (but before discharge) tends to disrupt the aim of the firearm.
A further disadvantage of open bolt firearms is the increased risk of a malfunction at a critical moment when a first shot is needed. This is because every step of firearm operation has some small percentage risk of failure, and open bolt firing requires not just that the cartridge properly discharge when struck by the firing pin, but that the cartridge be properly stripped, fed, and chambered, each of which has some risk of malfunction.
Closed bolt operation, on the other hand, suffers the risk of cook-off, but enjoys the advantages of accuracy and reliability. Accuracy is provided because the bolt remains stationary up until the trigger is pulled and discharging of the cartridge has occurred. Reliability is provided because the risks of feeding the cartridge may be undertaken before the critical moment, allowing any malfunction to be addressed before encountering a threat, and because the action is closed to keep out contaminants.
Therefore, closed bolt firearms are generally used for semi-automatic applications, while open bolt arms tend to be used for fully-automatic applications, where accuracy is less critical, and cook-off is a greater concern.
Firearms have been developed that employ both open and closed bolt operation modes. One example is the German FG-42, a World War II era machine gun that employed select fire operation (allowing a choice of semi and full automatic) and which fired from an open bolt position during full automatic fire, and from a closed bolt position during semi-automatic operation. A change lever engages one of two sears depending on the mode of fire selected.
Modern firearms have been disclosed that employ open and closed bolt operation. For instance, US Patent Application Publication 2007/0051236 to Groves et al. discloses a weapon platform that operates in open bolt mode on full auto, and closed bolt when the selector switch is set to semi-auto mode. This disclosure, incorporated herein by reference, discloses a feature of allowing the user to switch from open-bolt/full-auto mode to Semi-auto without the bolt closing, avoiding the noise of bolt closure, which may be disadvantageous in certain circumstances. Of course, this means that the rifle suffers the inaccuracy and other disadvantages of open bolt operation on the first shot from semi-auto after transitioning from open bolt auto operation.
The above disclosure suffers from several other disadvantages. First, it is not adaptable to update or improve the many existing lower receivers (which contain the fire control group, support a grip, and receive a magazine) for these arms, and requires that an entire new lower receiver be supplied. Second, the pivot pins employed are supported only by the relatively thin bodied aluminum, steel or polymer composite material used for the lower receiver. The stresses generated by the reciprocating bolt that interacts with the fire control components can damagingly stretch the holes that hold the pivot pins that support the components. Third, while the auto sear prevents discharge with the bolt out of battery, it has no safety effect to prevent out-of battery discharge when the rifle is in semi-auto mode.
The present invention overcomes the limitations of the prior art by providing a firearm having a frame with a barrel connected to the frame and defining a barrel axis. The barrel has a rear end defining a chamber, and a bolt assembly reciprocates with respect to the chamber between a closed position adjacent the chamber, and an open position away from the chamber. A fire control assembly includes a trigger and a selector switch with a semi-automatic position and a fully-automatic position. The fire control assembly includes a bolt assembly sear operably engaging the bolt. The fire control assembly operates when the selector switch is in the semi-automatic position in response to pulling the trigger to discharge the firearm, to load a cartridge, and to position the bolt in the closed position. The fire control assembly operates when the selector switch is in the fully-automatic position in response to pulling the trigger to discharge the firearm, and to hold the bolt in the open position. The fire control may include a sub-frame connected to the frame and to the bolt assembly sear, and may include a safety sear that prevents firing out of battery when in either full-auto or semi-auto modes. The fire control system may include a facility that momentarily maintains the trigger in a firing position when it is released while the bolt is moving forward from the open position.